The invention relates to chemical mechanical polishing of substrates, and more particularly to an article and method for polishing a substrate.
Integrated circuits are typically formed on substrates, particularly silicon wafers, by the sequential deposition of conductive, semiconductive or insulative layers. After each layer is deposited, it is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly nonplanar. This nonplanar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface to provide a planar surface. Planarization, in effect, polishes away a non-planar, outer surface, whether a conductive, semiconductive, or insulative layer, to form a relatively flat, smooth surface.
Chemical mechanical polishing is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head with the exposed surface of the substrate placed against a rotating polishing pad or moving polishing belt (both of which will be referred to herein as polishing pads). The polishing pad may be either a xe2x80x9cstandardxe2x80x9d pad or a fixed-abrasive pad. A conventional standard pad is formed of a durable material, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad.
A polishing slurry, including at least one chemically-reactive agent (e.g., deionized water for oxide polishing), and abrasive particles (e.g., silicon dioxide for oxide polishing) if a standard pad is used, is supplied to the surface of the polishing pad. The slurry can also contain a chemically reactive catalyzer (e.g., potassium hydroxide for oxide polishing).
One conventional polishing pad, described in U.S. Pat. Nos. 5,578,362 and 5,900,164, is a hard composite material with a roughened polishing surface. This polishing pad is composed of solid cast block of durable urethane mixed with fillers, such as hollow microcapsules, which provide the polishing pad with a microporous texture. The polishing pad has a low compressibility, is plastically deformable, and has a relatively low tensile modulus. This polishing pad is available from Rodel, Inc., located in Newark, Del., under the trade name IC-1000.
Another conventional polishing pad, described in U.S. Pat. Nos. 4,728,552 and 4,927,432 is a soft composite material with a compliant polishing surface. This polishing pad is composed of a dense net or mesh of polyester fibers, such as Dacron(trademark), oriented substantially perpendicular to the polishing surface of the pad and leached or impregnated with urethane. The urethane fills a significant fraction of the void space between the fibers. The resulting pad is relatively compressible, is plastically and elastically deformable, and has a relatively low tensile modulus. This polishing pad is available from Rodel, Inc., under the trade name Suba-IV
A two-layer polishing pad, described in U.S. Pat. No. 5,257,478, has an upper layer composed of IC-1000 and a lower layer composed of SUBA-IV. The polishing pad may be attached to a rotatable platen by a pressure-sensitive adhesive layer.
Yet another conventional polishing pad, described in U.S. Pat. No. 4,841,680, is soft poromeric material with a compliant polishing surface. This polishing pad is composed of a urethane with tubular void structures oriented perpendicularly to the polishing surface to provide the polishing pad with a spongelike texture. The resulting pad is relatively soft, and has a relatively low elastic modulus. This type of polishing pad is available from Rodel, Inc., under the trade name Polytex.
A conventional fixed abrasive polishing pad includes discrete islands or blocks of polishing material formed on a multilayer sheet. The islands of polishing material are composed solid blocks of resin in which abrasive particles, such as silicon, aluminum or cerium particles, are dispersed. The resulting pad, although flexible, is relatively non-compressible and inelastic. As a substrate is polished, the resin is worn away to continuously expose additional abrasive particles. Fixed abrasive polishing pads are available from 3M, Inc., located in Minneapolis, Minn.
The effectiveness of a CMP process may be measured by its polishing rate and by the resulting finish (roughness) and flatness (lack of large-scale topography) of the substrate surface. Inadequate flatness and finish can produce device defects. The polishing rate sets the time needed to polish a layer and the maximum throughput of the polishing apparatus.
One limitation on polishing throughput, particularly when IC-1000 is used as the polishing material, is xe2x80x9cglazingxe2x80x9d of the polishing pad surface. Glazing occurs when the polishing pad is frictionally heated, shear stressed, and compressed in regions where the substrate is pressed against it. The peaks of the polishing pad are pressed down and the pits of the polishing pad are filled up, so the surface of the polishing pad becomes smoother and less able to transport slurry. As a result, the polishing time required to polish a substrate increases. Therefore, the polishing pad surface must be periodically returned to an abrasive condition, or xe2x80x9cconditionedxe2x80x9d, to maintain a high throughput. The conditioning process is destructive and reduces the lifetime of the polishing pad.
Another limitation on throughput is the lifetime of the polishing pad. If a polishing pad wears out, it needs to be replaced. This requires that the polishing machine be shut down temporarily while a new polishing pad is affixed to the platen. The typical lifetime of an IC-1000 polishing pad is about 400-800 wafers.
An additional consideration in the production of integrated circuits is process and product stability. To achieve a low defect rate, each substrate should be polished under similar conditions. However, the mechanical properties of a set of polishing pads can vary from pad to pad. In addition, changes in the process environment during polishing, such as temperature, pH, and the like, can alter or degrade the polishing pad, thereby leading to variations in the mechanical properties of the pad from substrate to substrate. This variability may lead to substrate surface variability.
Another consideration about conventional polishing pads is effective slurry transport. Some polishing pads, particularly pads with a solid non-porous polishing surface, such as the IC-1000, do not effectively or uniformly transport slurry. A result of ineffective slurry transport is non-uniform polishing. Grooves or perforations may be formed in a polishing pad to improve slurry transport.
In one aspect, the invention is directed to a carrier head that has a substrate receiving surface and a retaining ring surrounding the substrate receiving surface. The retaining ring includes a mesh of fibers and a binder material holding the fibers in the mesh. The binder material is coalesced among the fibers to leave pores in the interstices between the fibers of the mesh. The fibers and binder material provide a surface of the retaining ring with a brittle structure.
Implementations of the carrier head may include one or more of the following features. The fibers may include cellulose, e.g., linen, or a polyamide, e.g., Aramid. The binder may include a resin, e.g., a phenolic resin.
In another aspect, the invention is directed to a chemical mechanical polishing apparatus with a polishing pad and a carrier head that includes a retaining ring surrounding a substrate receiving surface. The polishing pad includes a first mesh that has fibers and a binder material to hold the fibers in the first mesh, and the retaining ring includes a second mesh with the fibers and the binder material to hold the fibers in the second mesh.
Implementations of the carrier head may include one or more of the following features. In the first and second mesh, the binder material may be coalesced among the fibers to leave pores in the interstices between the fibers. The fibers and the binder material may provide the first and second mesh with a brittle structure.
In another aspect, the invention is directed to a retaining ring that has a mesh of fibers and a binder material holding the fibers in the mesh. The binder material coalesced among the fibers to leave pores in the interstices between the fibers of the mesh. The fibers and binder material provide a surface of the retaining ring with a brittle structure.
In another aspect, the invention is directed to a chemical mechanical polishing apparatus. The apparatus has a polishing pad, a carrier head to hold a substrate in contact with the polishing pad, a port to dispense a polishing liquid onto the polishing pad, and a pad cleaner including a plurality of nozzles to direct jets of a cleaning fluid onto the polishing pad and a brush to agitate a surface of the polishing pad.
Implementations of the invention may include one or more of the following features. The pad cleaner may include a plurality of vacuum ports to suction cleaning fluid away from the polishing pad. The brush may be a rotating cylindrical brush. The polishing pad may include a mesh that has fibers and a binder material to hold the fibers in the mesh.
In another aspect, the invention is directed to a chemical mechanical polishing apparatus with a polishing pad, a carrier head to hold a substrate in contact with the polishing pad, a port to dispense a polishing liquid onto the polishing pad, and a platen to support the polishing pad. The polishing pad has a mesh of cellulose fibers and a phenolic resin binding the fibers in the mesh, the resin coalesced around the fibers to leave pores in the interstices in the fiber mesh. The platen includes one or more channels through which a coolant flows.
Advantages of the invention may include one or more of the following. The polishing pad can be fabricated using techniques that are conventional in the automobile clutch and brake pad industry, and can have a low manufacturing cost. The polishing pad can have an intrinsically long lifetime, and may not need conditioning. This also permits the polishing apparatus to be constructed without a conditioner apparatus, thereby reducing the cost and complexity of the polishing apparatus. If the polishing pad is conditioned, it can be conditioned with another piece of polishing pad rather than a diamond-coated disk, thus reducing the cost of the conditioning device. The polishing pad can provide uniform material properties as it is worn away, thus providing a uniform polishing rate throughout the lifetime of the pad. The polishing pad is unlikely to cause scratching of the substrate. The polishing pad can be wetable and can effectively transport slurry without grooves or perforations. The polishing pad can be mounted to a platen without a subpad. The polishing pad can be thermally stable over a wider range of temperatures than conventional pads, thereby improving polishing uniformity. The polishing pad can be formed with a roughness or surface friction sufficient to provide a satisfactory polishing rate.
Additional features and advantages of the invention will become apparent from the following description including the drawings and the claims.